This invention relates to a control system for regulating the heating of water utilized to humidify oxygen gas being administered through an inhalation therapy apparatus.
The introduction of heat to humidified oxygen through inhalation therapy apparatus is generally well known as disclosed for example, in U.S. Pat. No. 3,903,883 to Pecina et al. The heating control system in such apparatus includes a temperature probe mounted in the inhalation tube, through which the humidified oxygen is conducted, to sense the temperature therein at a site adjacent entry into the patient, somewhat downstream of the heater location. The temperature measurement signal of the probe is fed to the heater control system in an effort to so regulate the amount of heat applied that the temperature of the humidified oxygen as it enters the patient is maintained constant at a manually selected level. Further, according to the Pecina et al patent a second temperature sensor is mounted on the heater block so as to monitor the heater temperature and through the control system prevent overheating of the heater.
Because of the spacing between the heater and the temperature probe site adjacent the patient, the flowing nature of the medium being heated, and the thermal inertia involved, a temperature stability problem arises for inhalation therapy apparatus of the foregoing type within the operating range of selected temperatures and variations in flow conditions anticipated. In particular, heating control systems heretofore effective to regulate heating so as to attain a desired selected temperature, would often overshoot the selected temperature and result in excessive temperature swing or "hunting". The extent of such temperature instability would of course vary dependent on variations in flow conditions of the humidified oxygen.
It is therefore an important object of the present invention to provide an improved heating control system for inhalation therapy apparatus wherein temperature instability and temperature overshoot heating is minimized despite anticipated flow condition variations.
A further object in association with the preceding object of the invention is to provide an improved heating control system through which a plurality of alarm conditions may be detected and heater shut-off automatically effected to avoid apparatus damage and patient injury.
The invention of the present matter has been found to be especially efficacious when used with a system as disclosed in issued U.S. Pat. No. 4,110,419 herein incorporated by reference. There is therein disclosed an ingenious cartridge humidifier. The patentee discloses an elongated generally cylindrical cartridge. It has a bottom cap and a top cap, both of which may be constructed of plastic. The two caps are joined by a cylindrical body portion which, to obtain good heat exchange properties, is constructed of a metal, such as aluminum. An electrical heater is provided which has a large bore therethrough into which the aforementioned cartridge is positioned and retained. Internally with respect to both the bottom cap and the cylindrical body portion cylindrical capillary active member is provided with a diameter sufficient to be in essential contact with the inner wall of the body portion whereby the capillary active member is in excellent heat exchange position with the heater. While the cartridge may be supplied with a fixed quantity of water for humidification it has been found to be particularly efficacious to supply water in a replenishable manner from a supply reservoir which may be an inverted bottle of water. The bottle and the heater may be suitably bracketed together whereby the entire thusly constructed unit may be suitably mounted. The bottom of the cartridge and the bottle containing a supply of water are fluidly connected by suitable egrees and ingress ports and a connecting tube to provide an inverted siphon to continuously supply water from the bottle to the cartridge as it is removed therefrom. The water in the cartridge wets the capillary active member to thereby provide a larger area for evaporating water than would be possible if the member were not present. The upper cap is provided with a concentrically located tube to which a conduit supplying oxygen or air under pressure is supplied. The said tube has a downwardly depending extension that extends concentrically into the mentioned body portion. The top cap is provided with an egress port which is located transverse to the axis of the cartridge. The port is connected to flexible conduit means which is designed to distribute the humidified gas to a patient. The egress port is in communication with an annular space around the mentioned downward extending tube of the upper cap. In this manner the gases are first directed downwardly through the said tube and then upwardly through the mentioned annular space to the egress port.
Other patents of interest in practicing the present invention are U.S. Pat. Nos. 4,172,105 and 4,195,044, which deal with further modifications and are also incorporated herein by reference.